why I can't lift up the scissors' stucture

I have built my scissors’ structure, and I can use hand to lift it up, but when I turn on the power, the motors seems like not enough power. I use two 393 motors and gear ratio is 12 to 84. The battery is full energy.

I see no elastics being used in your lift at all. Try tuning your scissor lift with some elastics, and see if that helps. It could also be there’s a lot of friction present in your scissor lift, which you could minimize by, again, elastics, and possibly grease. As a last resort, try replacing the motors and/or motor controllers.

Did you lose track of your original posting? There were some suggestions provided there. I can’t tell from your photos but does the base of your scissor lift have metal sliding directly on metal? If so, is there some way to use rollers? or plastic sliding on metal to reduce friction?

I agree with Space that elastics are very helpful in designs like this. :slight_smile:

Maybe take a look at your original post: https://vexforum.com/t/how-to-hold-the-position-when-i-lift-up-something/26886/1

Well the piece you are sliding your scissor across has a lot of friction. Pretty much all the weight of your lift makes the axle stab into the metal “slide” and causes more and more friction. Metal on metal is something to be avoided if possible.

Next your whole lift looks like its made of steel.

Next your lift appears to use axles and bearings everywhere. This causes more friction than any other type of joint in VEX.

Next your axle supporting the bottom of the scissor seems to be very far away from the “slide”. This means the axle is bowing down a little bit causing even more friction.

Next every joint along the scissor only allows the metal to be supported on one side allowing the metal to sag down and cause more friction.

Our good friends TVex (9090C) had a scissor-lift powered at the center just like yours–but they used 4 motors (393s). Our scissor-lift on Floyd II (400X) was powered from the bottom, but it also used 4 motors AND the old-type sliders. I’m pretty sure you’re going to have to use 4 motors with your current design. You’ll also need to add another tier and follow the previous suggestions regarding friction-reduction, weight reduction and the use of elastics. Best of luck to you!

That got me thinking. Could you please explain why bearing and axle have the greatest friction? What do you think reduces friction?

As I am designing a scissor also, few joint types come to my mind: bearing and axle, axle directly in square hole, screw joint with bearings, screw joint without bearings. Which one would you prefer?

Direcrly replying the thread, I think that scissor is one of the hardest things to perfect in VEX. It is easy to build one; it is hard to make it perform as you wish. My suggestion is to do more research and look at world level VEX competition scissor lifts, like TVEX or 400x. Spend more time designing and think about more factors that add friction, and improve your design and build quality overall.

A lot of the aforementioned stuff, like putting rubber bands on your lift, will help.

I might try putting bearings on the other side of the 12-tooth gear, even if it means taking off the bearing on the motor side.

I personally prefer using stiffer connections (a c-channel, for example) to connect the two sides of the scissor lift because I find that axles and standoffs provide inadequate support.

First thing I’d do is make sure your motors are trying to turn in the same direction. I’ve made that mistake a few times…

How many types of joints are there? Hinges have more friction in my experience.

Use screws (with lock nuts so they don’t unscrew) and bearing blocks instead of axles and bearing blocks for joints such as these. Always make sure you have at least 1 washer between two pieces of metal that are sliding against each other. Additionally, removing the axles and collars will reduce the weight quite a bit.

As most people have stated, try to get as much friction out of the lift as you can. Pull the axles out of the motors so you can move it freely by hand to test and adjust. Elastics can also help a lot to reduce the strain on the motors.

Also, take out your motors and test them, make sure they are fine.

We had a very effective and fast 2 motor scissor lift for Toss Up, and I have had a lot of experience with them having been pretty much the only one who built it. It will be frustrating at times, but when it works it’s great.
A scissor lift is relatively easy to build, but virtually impossible to perfect.

We had a very effective and fast 2 motor scissor lift for Toss Up, and I have had a lot of experience with them having been pretty much the only one who built it. It will be frustrating at times, but when it works it’s great.
A scissor lift is relatively easy to build, but virtually impossible to perfect.

Now I’m trying to use worm gear to instead of 12-tooth gear in order to increase the lift power and avoid descend automatically. Could you please show me how do you bulid your scissor lift. My email is minhaonian@gmail.com. Thank you.

Until team 472 responds in full, you might have a look at their website for a little information:

I have been wanting to do this since Worlds, this weekend I will put together a post with all the different things we found out about scissor lifts. Once I find some pictures and put that together, I can also email it to you.

I recall doing some testing with worm gears for our Toss Up robot around December/January. I remember our lift being really slow, but it certainly was effective in making sure that our lift stayed up. I think we scrapped the idea due to the speed of the lift, and how much “clicking” it did, but it was an interesting experience to try and build it.

I’m guessing you will find the worm gear to be slower than you wish. It is true that worm gears are excellent at preventing “backdrive”. In other words, the load can not push back and force the motor to run backwards when the motor has no power to it. However, on the other hand, worm gears are notorious for having lots of friction, which consumes a lot of your power when you are driving it.

Bottom line: worm drives are slow and eat up a lot of power. Just keep that in mind.

Sorry guys, I didn’t have any time this weekend to work on it, so it will have to wait a little bit. I also discovered that I have surprisingly few pictures of our robot, so I might have to wait until school starts again to do this.

More power!!!

It looks as if you’re using the regular steel parts. They are heavy. Use two more motors, one for each side of the lift.

Also, as mentioned, use elastics.

In our experience, it is the first 15% or so of the scissors lift going up that is the most difficult to overcome. The reason for that is you’re trying to translate a ton of force to make them slide parallel to the ground into upward motion.

If your lift doesn’t have to go all the way down, this will save considerable effort for the motors. Elastics will help.

I think the reason that you can lift it up with your hand but the motors won’t do it is because your hand is stronger than the motors. Loosen the structure with a bit of lubricant and use elastics. It will make a difference

Sorry for those that have heard this one before in the ten other posts I may have put this.

The placement of the “pusher” on the scissor to get it to lift has a big effect on the force required to lift the scissor. How you have your lift will mean different types of forces required to start the lift.

Referenced in vamfun’s blog:

This is the slightly dry document that may be a bit of a difficult read…

Pages 47-49 are the important ones. (pages 53-55 of the PDF). Notice how some placements of the pusher actuator makes the forces go to darn near infinity? That means it is really hard to get the scissor started.

So you may be starting too flat. 10-15 degrees should be good enough of a start angle and the rubber bands kick in on the initial push.

Redesign the lifter part is what I would suggest and ensure the scissor without any lifter hookup is easy to get going with rubber bands. Get out as much friction and wobble as you possibly can. THEN add the lifter mechanism and tune that since you have now minimized the forces needed to start lifting.

Engineering and math win again folks…:cool:](“http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA225220”)